net: respect resolv.conf rotate option

Instead of ranging over servers in the config, grab an offset
from the config that is used to determine indices.

When the rotate option is enabled, the offset increases which
rotates queries through servers. Otherwise, it is always 0
which uses servers in config order.

Fixes #17126

Change-Id: If233f6de7bfa42f88570055b9ab631be08a76b3e
Reviewed-on: https://go-review.googlesource.com/29233
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/net/dnsclient_unix_test.go

@@ -797,3 +797,73 @@ func TestRetryTimeout(t *testing.T) {
 		t.Error("deadline0 still zero", deadline0)
 	}
 }
+
+func TestRotate(t *testing.T) {
+	// without rotation, always uses the first server
+	testRotate(t, false, []string{"192.0.2.1", "192.0.2.2"}, []string{"192.0.2.1:53", "192.0.2.1:53", "192.0.2.1:53"})
+
+	// with rotation, rotates through back to first
+	testRotate(t, true, []string{"192.0.2.1", "192.0.2.2"}, []string{"192.0.2.1:53", "192.0.2.2:53", "192.0.2.1:53"})
+}
+
+func testRotate(t *testing.T, rotate bool, nameservers, wantServers []string) {
+	origTestHookDNSDialer := testHookDNSDialer
+	defer func() { testHookDNSDialer = origTestHookDNSDialer }()
+
+	conf, err := newResolvConfTest()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conf.teardown()
+
+	var confLines []string
+	for _, ns := range nameservers {
+		confLines = append(confLines, "nameserver "+ns)
+	}
+	if rotate {
+		confLines = append(confLines, "options rotate")
+	}
+
+	if err := conf.writeAndUpdate(confLines); err != nil {
+		t.Fatal(err)
+	}
+
+	d := &fakeDNSDialer{}
+	testHookDNSDialer = func() dnsDialer { return d }
+
+	var usedServers []string
+	d.rh = func(s string, q *dnsMsg, _ time.Time) (*dnsMsg, error) {
+		usedServers = append(usedServers, s)
+
+		r := &dnsMsg{
+			dnsMsgHdr: dnsMsgHdr{
+				id:                  q.id,
+				response:            true,
+				recursion_available: true,
+			},
+			question: q.question,
+			answer: []dnsRR{
+				&dnsRR_CNAME{
+					Hdr: dnsRR_Header{
+						Name:   q.question[0].Name,
+						Rrtype: dnsTypeCNAME,
+						Class:  dnsClassINET,
+					},
+					Cname: "golang.org",
+				},
+			},
+		}
+		return r, nil
+	}
+
+	// len(nameservers) + 1 to allow rotation to get back to start
+	for i := 0; i < len(nameservers)+1; i++ {
+		if _, err := goLookupCNAME(context.Background(), "www.golang.org"); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	if !reflect.DeepEqual(usedServers, wantServers) {
+		t.Fatalf("rotate=%t got used servers:\n%v\nwant:\n%v", rotate, usedServers, wantServers)
+	}
+}